Although the stator windings of electrical inductive devices, such as motors, are wound with magnet wire having enamel or other insulative coating thereon, it is often desirable to further coat the windings and seal them from the environment. When the motor is used in environments where the stator windings are exposed to moisture or abrasive materials, such as sand and dirt, it is often desirable to further protect the stator windings from the environment by means of an additional coating. For example, protection of the stator windings is desirable in blower motors utilized in the cooling systems for locomotive traction motors. Protection is also desirable in open motors utilized in driving pumps in oil field applications, which are exposed directly to blowing sand and dirt, as well as moisture.
Conventional curable varnish compositions, such as those used in certain locomotive traction motors, are so-called “solventless” varnishes based on unsaturated polyester resin (UPR). These varnish systems, however, have a glass transition temperature (Tg) significantly below 80° C. Thus, their performance at motor operating temperatures, usually about 160° C., can result in significant thermal degradation after extended operating times. In addition, such varnishes tend to chip or crack, particularly when subjected to vibrations accompanying locomotive operation. Such UPR varnishes also have a high moisture absorption rate and their ester bonds are hydrolysable, which may result in more frequent maintenance intervals than desired.
What is needed is a varnish composition that can better withstand higher temperature and a method for electrically insulating electrical devices with the varnish composition.